TDS 500, 600, 700 series NVRAM battery replacement

[Polarue]

I posted an idea to solve the aging LTC battery issue that all TDS scope owners with the internal battery NVRAM Dallas chips have due to their age.

  It is simply to put an external battery on the Vcc leg (#32) of the nvram chip.  From the datasheet of the ds1486-120, the internal switch does not connect the LTC battery until the voltage on Vcc drops to about 3v.  A lithium ion rechargable battery would work if some carefully chosen diodes were to allow the roughly 5v Vcc input to charge the lithium ion when the unit was on to no more than 4.2v and then another diode would only allow about 3.7 or more volts to flow when the unit was turned off.  Once in place, the Vcc would never again fall low enough to switch over to the failing LTC battery if the unit was on every three months or so.

Does anyone with more knowledge see any fatal flaws with this modification to extend the life of the nvram chip's data?

[Polarue]

"As VCC falls below approximately 3.0V, a power switching circuit turns the internal lithium energy source on to maintain the
clock and timer data functionality. It is also required to ensure that during this time (battery-backup
mode), that the voltage present at INTA and INTB (INTB) never exceeds VBAT. During power-up, when
VCC rises above VBAT, the power-switching circuit connects external VCC and disconnects the internal
lithium energy source. Normal operation can resume after VCC exceeds 4.5V for a period of 200ms."

The above is from the ds1486-120 datasheet.  I believe Vcc is normally around 5v so a circuit could be built that allows 4.2v to charge the external battery while the unit is on and then blocks the current to the rest of the board when the unit is off (when the external battery provides 3.7v to the chip). This would require the pin32 (Vcc) to be lifted and integrated with the simple battery circuit.

Can anyone say why this would not work?